Name of Material

Table of Contents

• Table of Contents

• Introduction

• Problem

• Solution Concept

• Master Table

• Design Reasoning

• Material Selection

• Testing

• Construction

• Progress Account

• Conclusion

Introduction

This document is a detailed report of the designing of an outdoor seat for a school engineering technology assignment. Our team consists of four students and our project was part of a larger project involving every student in the class, aimed at improving the outdoor area adjacent to our classroom.

Structural engineer and model builder

Problem

The initial problem addressed by our engineering class was the unexciting state of an area outside our engineering classroom, between the manual art and science blocks, referred to as the engineering foyer. The entire class was to work together to transform this ordinary area into an innovative relaxation area for senior students, including shelter, seats, tables, drainage, landscaping and even a bridge. All these elements were to share an engineering theme and come together to form a detailed overall plan. After all students put forward their ideas, the class was split into a number of teams. Our team was assigned the task of designing the seating.

Our problem at that stage was to design a fixed outdoor seat, which was ergonomic, economical, original, secure, resistant to natural elements and could be placed next to a table without making it uncomfortable for students to walk between the two. Furthermore, we needed to perform physical and theoretical testing on our design, construct a model and design a system for efficient production.

Solution Concept

Our solution to the problem was an original design we referred to as a fold down outdoor seat. The main feature of this seat is the base of the chair, which is weighted at the back and pivoted so as to flip upright when no one is sitting on it (See Figure 1). Aside from adding complexity to the design, which satisfied our teacher, this characteristic serves as a space saving feature, which allows students to walk easily between the seat and the table. This feature was important to include, despite the additional expenses involved, because it was originally planned to place a retaining wall directly behind each of our seats with insufficient space to pass through (See Figure 2). We decided that the backrest surface of the chair would also be weighted and pivoted to make the seat more comfortable. Either end of the seat would be supported with a solid beam penetrating into a concrete foundation in the ground.

Note: Nine of these seats, three to fit around each of the three tables, were required therefore the cost of producing nine seats was calculated and their mass production considered.

1

Master Table

Specifications

Price

Required Length

Total Cost

One Seat

• Total cost of one seat – $130
• Total cost of nine seats – $1090 ($121 per seat, due to commercial lengths available)

Note: 15% price reduction of purchases made by schools not included. All prices are pre GST.

1

Design Reasoning

Overall Design

Our seats needed to be positioned around a hexagonal table of 1.4m diameter so we decided to design the seat for two people, because a three-person seat would be too long and a one-person seat was simply impractical (See Figure 3). After looking at a number of seats, the length of the base and backrest (1.2m) was based on the general sitting space allocated for each person (60cm). The height above ground level to the base (50cm) and backrest (90cm to the highest point) was based on an average model of a senior student (See Figure 4). The width for the base (approximately 40cm to the pivot point) and backrest (30cm) were largely based on the design of other chairs.

The poles recede 25cm into the ground for stability and are centred within a cylindrical shaped concrete foundation. It was intended to calculate the required size for these foundations, but as we were unable to we estimated them at 20cm diameter and 30cm deep.

Main Poles and Concrete Foundations

The stability of the two main poles on either side of the seat was essential. We choose to extend the length of the poles above the backrest pivot rod to horizontally in line with the top of the backrest (90cm above ground) so we could later place project stoppers from the pole to limit the rotational movement of the backrest. These two poles, originally vertical were tilted back slightly (12.5o receding a horizontal distance of 20cm) so that the foundation would be roughly centred underneath the weight of a seated person (See Figure 5).

Pivot Rods

In order for the base of the seat to fold down, a pivot axis was required. One option was to use two pivot mechanisms affixed to either end of the base and the neighbouring main pole. However it was decided that one pivot rod, extending from one main pole to the other and therefore penetrating the support bar in the middle of the base frame, would have greater stability (See Figure 6). The same option was chosen for the backrest for consistency and efficient production.

Support Bars

A support structure was necessary to affix the wooden planks onto and support people’s weight on the backrest and base in particular. Taking dimensions into account, a figure of eight design was the best option so as to form two equal squares (See Figure 6). The middle beam separates two people using the chair properly and therefore any depression of the planks caused by one person would be reduced and fail to affect the other person. The theory for the backrest was the same.

Planks

Through default the base and backrest surfaces of the chair were wooden planks. We separated each of these planks by 10mm to allow for expansion without reducing comfort or causing a pinching affect. The planks were positioned lengthwise to reduce construction work and in the end four were required for the base and three for the back. To affix the planks to the support bars we decided to use small screws, five for each plank with two either end and one centred in the middle.

Stopper Bars

The fold down base of the seat needed to come down to rest in a horizontal position and therefore required a mechanism to support it. Furthermore the torque would need to be shifted off the relatively weak base pivot rod when the chair was in use. To solve these problems two solutions were recognised. The first was to use a length of chain affixed from the main pole to the base, which would work in tension when the chair was in use and collapse when the chair fold. Unfortunately a chain attached to the front of the base would restrict the movement and potentially cause discomfort for students, and the lengths of chain would need to be exact on both the front and back side of the base in order to prevent torsional forces acting in shear on the base pivot rod (See Figure 1).

The second solution was to use something solid to act in compression. We chose this solution and used two accurately positioned continuous lengths of beam to prevent the shear on the pivot rod (See Figure 5). However because, unlike the chain idea, the bars needed to be affixed to the intersection between the main poles and the base, the torsion on these bars was found to be massive. For the top beam an angle bar was chosen as it would not conflict with the movement of the backrest. The bottom beam needed to be extremely supportive of the base support structure between both main poles while the chair was in use, so a wide flat bar was chosen. In the case either of these deformed significantly in the middle, we could always weld strategically placed small flat bars to support it later.

Note: It was recognised that the stopper bars could introduce a safety hazard. To account for this, small flat plastic hand guards could be position either end of the base support bar structure so it would be impossible to wedge a finger between the base and bottom stopper while the base is lowered down. Also a plastic lining could be easily glued after the construction of the seat to prevent the noise and confliction of metal surfaces when the seat is lowered and wooden surfaces when the seat falls shut.

Material Selection

Main Poles, Pivot Rod, Support Bars – Galvanized Steel

• Stainless Steel – Expensive
• Titanium – Far too expensive
• Grey Cast Iron – Heavy and rough
• Wood – Lower strength to density ratio, decays rapidly.
• Mild Steel – corrodes unless painted

• Economical
• High strength to weight ratio

• Moderately hard, tough and durable

• Average to long life
• High tensile strength
• Resistant to corrosion

• Heavy in weight
• Moderately expensive

• Pine – Considered too expensive despite wonderful wood grain patterns

• Strong
• Fairly attractive
• Difficult to flex
• Relatively inexpensive
• Weather resistant

• Ability to burn
• Moderately expensive

• Stainless Steel – Expensive
• Titanium – Far too expensive
• Grey Cast Iron – Heavy and rough
• Wood – Lower strength to density ratio, decays rapidly.
• Galvanized Steel – Slightly more expensive

• Economical
• High strength to weight ratio

• Moderately hard and tough

• High durability
• Average to long life
• High tensile strength

• Heavy
• Eventual corrosion unless painted

Testing

Overall Design

As a principle of engineering, it is always necessary to account for the worst-case scenario. In testing the seat we accounted for the force of two particularly stupid senior students jumping at once on the end of the seat, which is only possible if a third student was holding the seat down.

Note:

Force = Mass × Gravity

• Average mass of senior student = 70kg
• All results were derived using scales and averages
• Force is distributed differently depending on how a person sits in a seat

Main Poles and Concrete Foundations

Note:

Moment = F × d

• Seat has two main poles

Two Students Jumping

Moment Bottom = (3300 × 0.3) ÷ 2

Moment Bottom = 495 Nm Clockwise

Force Resultant = 495 ÷ 0.25

Force Resultant = 1980N Each Pole

Two Students Sitting

Moment Bottom = [(210 × 0.9)  (620 × 0.3)] ÷ 2

Moment Bottom = 1.5 Nm Anticlockwise

Force Resultant = 29.5 ÷ 0.25

Force Resultant = 6N Each Pole

Note: Neither the required dimensions of the cylindrical concrete foundations or the tilted circular section main poles could be derived using known formula and physical testing was impractical. However these torsional equations provided us with a rough idea of what we needed and the pole was well centred underneath the weight of a student.

Support Bars

Note:

Stress = My ÷ I

M = Fd

y = ½ b1

I = I Outside I Inside = (b1h13 ÷ 12)  (b2h23 ÷ 12)

Stress = (Fd × ½b1) ÷ [(b1h13÷12)  (b2h23÷12)]

• Stress maximum for steel = 250MPa
• Seat has three short base support bars

Box Section 2.5×25×25mm

Stress Total = (3300×0.4 × ½×0.025) ÷ [(0.025×0.0253÷12)  (0.02×0.023÷12)]

Stress Total = 859MPa

Stress Each Beam = 8.59×108 ÷ 3

Stress Each Beam = 286 MPaWill deform

Box Section 2.5×40×40mm

Stress Total = (3300×0.4 × ½×0.04) ÷ [(0.04×0.043÷12)  (0.035×0.0353÷12)]

Stress Total = 299 MPa

Stress Each Beam = 2.99×108 ÷ 3

Stress Each Beam = 99.7 MPaWill not deform

Safety Factor = 2.5×108 ÷ 9.97×107

Safety Factor = 2.5

Physical Testing

A length of unused box section 2.5×25×25mm was found and a simple but practical test applied. Rather than the force of two 70kg students jumping on three beams we used a 50kg student jumping on the end of one beam supported over the edge of a concrete slab by a number of others on a plank. According to our calculations, the bar bent considerably during the test and deformed massively when a heavier person insisted on having a turn. In retrospect we could have bought a number of different types of box section and tested them the same way with increasingly heavier people. Our results could then establish a safety factor and the distance the beam flexed.

Note: Box section 2.5×40×40mm was chosen with a high safety factor of 2.5 in the worst-case scenario. This type of box section was recognised as bulky and fairly expensive, however the nearest size down available to use commercially would have deformed.

Pivot Rods

The dimension for pivot rods was one of the last things to work out because its size was dependant on the width of the box section support bars. Testing was not really possible, but we decided to make the base pivot rod as large as possible so, if during the construction of the seat the stopper bars were poorly positioned, the rod could handle a decent portion of the applied force of a student without deforming.

Planks

A physical test could easily have been performed to decide the dimensions of the planks, however this would have cost us money. A company assured us that kwilla planks of 1990mm affixed to our structure would not break when students sat or stood on the seat provided the unsupported span was less than 1 metre.

Stopper Bars

Note:

Moment Clockwise = Moment Anticlockwise

Two Students Jumping

Force Top Angle Bar = (3300 × 0.35) ÷ 0.15

Force Top Angle Bar = 7700 N

Force Bottom Bar = (3300 × 0.45) ÷ 0.15

Force Bottom Bar = 9900 N

Two Students Sitting

Force Top Angle Bar = (620 × 0.35) ÷ 0.15

Force Top Angle Bar = 1447 N

Force Bottom Bar = (620 × 0.45) ÷ 0.15

Force Bottom Bar = 1860 N

Note: These calculations show some extremely large resultant forces applied to the top and bottom bar and it was frustrating to realise how accounting for the stupidity of students made our task difficult. Unfortunately we knew of no way to calculate the required dimensions of these two bars so instead we ‘guessed big’. It was decided to use a 5150mm flat bar for the bottom beam and 35050mm angle bar for the top beam. The teacher and retail outlet assured us this angle bar was impossible to bend and the flat bar would be sturdy although its tilted nature might cause slight deformation in the middle. Both these bars would be welded directly onto the main poles.

Construction

Note: This section includes a process for the production of the fold down outside seats.

Step One

Mark out and cut all materials to their appropriate sizes according to the table below using the method indicated. For mass production design a thin metal ruler from a piece of flat bar with all relevant measurements accurately marked and labelled.

• 21.15m

• 21.4m

• 21.2m with 45o angles
• 20.6m with 45o angles
• 10.52m

• 21.2m with 45o angles
• 20.3m with 45o angles
• 10.22m

• 71.2m

• 11.4m

• 11.4m

Step Two

Into one of the 1.2m base support bars pour in the same cement used for the seat foundations. This bar needs to be positioned at the back of the base support bar structure and its weight will cause the seat to flip upright.

Step Three

Weld together steel base support frames. For mass production build a special wooden mould. The mould will consist of a large wooden sheet base marked out with measurements for both base and backrest support bar structures overlapping. Piece together wooden blocks at key points so the bars can be placed and stay in position ready to weld or affix planks to.

Step Four

Using the mould and a heavy-duty drill with a 3.5mm drill piece screw the planks onto the base and backrest support bar structures. For each plank place two screws either end and one in the centre, sand and then finish.

Step Five

Using a heavy-duty drill with a 16mm drill piece, drill two holes into each main pole, and holes through the support bar structures for the two pivot rods. If possible use the same mould to establish the correct position for each hole.

Step Six

Weld the two stopper beams into position between the two main poles. Check the base support structure will fit and rotate correctly into position. The top angle bar, unlike the bottom flat bar, will need to be welded on a 12.5o angle so that it will be perfectly horizontal when the seat is finished.

Step Seven

Assemble together the support bar structures, main poles and stopper beams (See Figure 7). Next insert the two pivot rods through the holes and after confirming the seat is in working order, weld the rods at the ends to the main poles. Grease the moving surfaces well.